I tried getting access to the site once but gave up as the invitiation thing was a bit of a gatekeeper for me (by design)
If the invitee turns out to be a spammer, or posts a bunch of hate speech etc, the inviter may be held responsible for the invitee's actions. This isn't a hypothetical, it's explicitly explained in on the invite form, and I've seen cases where people were banned because they invited people who then acted spammy.
I'm not against the invite-only system, having been a forum moderator for several years, but I'm happy to just be a consumer/observer of the conversation rather than a participant.